According to a BBC article, researchers had theories about the mollies’ mustaches but only recently started studying them closely. Because the mollies have “a complicated mating behaviour, with males fertilising females internally, rather than spreading sperm over externally laid eggs,” males need to make themselves more attractive to females. And this is where that showy mustache comes in.

Scientists believe that the males’ mustaches play an important part in the courting process; the males will use their mustaches to stimulate the females’ genitals. Unfortunately for the fish fellows, mustaches come in different sizes, a very unlucky bunch of males have no mustache to speak of at all. When researchers studied a large group of mollies in a tank, they noticed the “females consistently preferred males with moustaches.” Color me surprised.